Filtration Leukoreduction Followed by Pathogen-Reduction (Mirasol Treatment) Prevents Alloimmune Platelet Refractoriness in a Dog Platelet Transfusion Model

The largest transfusion (tx) trial to evaluate methods of preventing platelet (plt) alloimmunization (TRAP Trial; NEJM 1997;337:1861) demonstrated residual alloimmunization rates of 17% to 21% in AML patients (pts) undergoing induction chemotherapy despite receiving either filter-leukoreduced (F-LR) or UV-B irradiated (UV-BI) blood products, respectively. Our pre-clinical dog plt tx studies, the basis for testing UV-BI in the TRAP Trial, demonstrated this model was able to predict pt results; i.e., prevention of alloimmunization was 45% in the dog but 79% in pts. The greater effectiveness in pts was probably because they had chemotherapy-induced immunosuppression compared to the immunocompetent dogs. Our current dog plt tx studies have focused on evaluating F-LR to remove antigen-presenting WBCs (APCs) or pathogen-reduction (PRT) (Mirasol treatment) to inactivate APCs.

For pts, plts are obtained using either apheresis procedures or as plt concentrates prepared from whole blood (WB). To re-duplicate these types of plts in our dog model, we prepared plt-rich-plasma (PRP) from WB which would be equivalent to non-leukoreduced apheresis plts. The PRP was then either unmodified, F-LR, PRT, or the treatments were combined. Because the success rates were very poor with the single treatments of PRP (see table), the WB studies evaluated only combined F-LR and PRT treatments. In clinical practice, the treated WB would then be used to prepare a plt concentrate. The WB studies assessed either PRT of the WB followed by F-LR of PRP made from the WB or, conversely, F-LR of the WB using a plt-sparing filter (Terumo Immuflex WB-SP) followed by PRT of the WB and then preparation of PRP. After completion of all treatments, PRP from each study was centrifuged to prepare a plt concentrate, the plts were radiolabeled with ⁵¹Cr, injected into a recipient, and samples were drawn from the recipient to determine recovery and survival of the donor's (dnr's) plts. Dnr and recipient pairs were selected to be DLA-DRB incompatible and crossmatch-negative. Eight weekly dnr plt txs were given to the same recipient or until the recipient became refractory to the dnr's plts defined as ≤5% of the dnr's plts still circulating in the recipient at 24-hours post-tx following 2 sequential txs.

The table shows the percent of recipients who accepted 8 weeks of dnr plts and the total number of dnr plts and WBC injected. Using either filter, there was equal reduction in WBCs to 10⁵/tx. Acceptance of unmodified dnr plts was 1/7 recipients (14%), PRT 1/8 recipients (13%), PL1-B filter 1/5 recipients (20%), and PLS-5A filter 4/6 recipients (66%). None of these differences were statistically significant. In contrast, combining F-LR of the PRP followed by PRT of the PRP was effective in 21/22 recipients (95%), regardless of the filter used. WB studies showed dnr plts were accepted by 2/5 recipients (40%) when WB was first treated with PRT followed by F-LR of the PRP made from the WB. Conversely, if the WB was first F-LR followed by PRT of the WB, 5/6 (83%) accepted dnr plts; more of these studies are in progress.

Data are given as average ±1 S.D.

F-LR of PRP or WB followed by PRT of the same PRP or WB is highly-effective in preventing alloimmune plt refractoriness in our dog plt tx model. These data suggest that most of the APCs must be removed by filtration before PRT can eliminate the activity of any residual APCs. Based on the high rate of success of this combined approach in our immunocompetent dog model, similar results should be achieved in pts even those who are not immunocompetent as were the AML pts receiving chemotherapy in the TRAP Trial.

Slichter:Terumo BCT: Research Funding.